The present invention relates to a circuit breaker used to protect an electric circuit. In particular, it relates to a circuit breaker that increases an opening speed of a movable contact shoe upon high-current interruption.
FIG. 8 is a side view of a current-interrupting section, showing a conventional example of a circuit breaker of this kind. In this figure, a U-shaped fixed contact shoe 1 formed of a flat type conductor and an L-shaped fixed contact shoe 2 are arranged in each phase so as to be positioned opposite to each other, with a fixed contact point 3 being attached to each of the contact shoes. A strip-shaped movable contact shoe 4, whose both ends are bent slightly inwardly, has a pair of movable contact points 5 to contact the fixed contact points 3. When the movable contact shoe 4 is closed as shown in the figure, a contact spring 6 presses the movable contact shoe 4 against the fixed contact shoes 1 and 2 so as to bridge the same. When the movable contact shoe 4 is opened, an opening and closing mechanism (not shown) presses down the contact shoe 4 against the spring force of the contact spring 6.
The fixed contact shoe 1 is connected to a power-side terminal (not shown). The fixed contact shoe 2 is lead to a load-side terminal through an overcurrent trip apparatus (not shown). A pair of arc-extinguishing chambers 7 is located, i.e. one in front and one at the rear of movable contact shoe 4, and a plurality of grids 8 for the chambers 7 surrounds the respective ends of the movable contact shoe 4. The grid 8 comprises a U-shaped magnetic plate, which is supported on side walls 9 made of a laterally disposed pair of insulators. In addition, an arc transition plate 10 made of a high-resistance material, such as steel plate, is provided to extend across the chambers 7, also serving as a supporting plate for the contact spring 6.
As shown by arrows, when a high current flows through the conducting path in FIG. 8, an electromagnetic repulsion force is generated between the parallel conductors of the fixed contact shoes 1 and 2 and the movable contact shoe 4, so that the movable contact shoe 4 is opened to the position indicated by the chain line (two dots) against a contact spring 6. An arc 11 is generated between the contact points 3 and 5. The arc 11 is driven by the grids 8 up to the tips of the fixed contact shoes 1 and 2 and the movable contact shoe 4, and is then extinguished quickly in the arc-extinguishing chambers 7. In this case, if the apparatus has an arc transition plate 10, as shown in the figure, a side leg of the arc 11 at the side of the movable contact shoe 4 moves to the plate 10, and the arc 11 is then extinguished with no current flowing through the movable contact shoe 4, thereby preventing the damage to the contact shoe 4 due to high current.
FIG. 9 is a side view of a conventional example in which an electromagnetic force acting on the movable contact shoe 4 during high current flow is raised further over the electromagnetic force of FIG. 8. That is, in FIG. 9, a pair of U-shaped magnetic plates 12 spaced in the longitudinal direction is located on the side of the movable contact shoe 4 opposite to the fixed contact shoes and the tips of their respective legs sandwich the movable contact shoe 4 in the closed position with a gap from the respective sides. This principle can be explained as follows with reference to FIG. 10. FIG. 10 shows a cross-section taken along line 10--10 in FIG. 9.
In this figure, if a current I flows through the movable contact shoe 4 in the downward direction of this sheet, a magnetic flux .PHI. based on the current I converges in the magnetic plate 12 and passes clockwise through the movable contact shoe 4 and the magnetic plate 12, as indicated by the chain-line arrow in FIG. 10. At this point, the magnetic flux .PHI. passes through the movable contact shoe 4 from left to right in FIG. 10 and flows perpendicular to the current I, so that a force is generated in the movable contact shoe 4 in the direction indicated by the solid arrows as a consequence of Fleming's left-hand rule. The movable contact shoe 4 is thus opened and driven at a speed higher than that produced by the electromagnetic resiliency alone in FIG. 8, to thereby improve the interrupting performance.
As described above, an electromagnetic force is conventionally used to drive the movable contact shoe in order to increase the opening speed and improve the circuit breaker performance.
An object of this invention is to make use of a different electromagnetic action to further increase the driving force for the movable contact shoe in order to further improve interrupting performance.